Direct I/O

I am running PostgreSQL 8.3.9 on Red Hat 4.4 with only 4GB of memory.  I would like to increase buffers but the problem is that I don't see much free memory. There is quite a large consumption of FS buffers:

            total       used       free     shared    buffers     cached
Mem:       4148528    3924496     224032          0      11912    3792940
-/+ buffers/cache:     119644    4028884
Swap:      8385920     997620    7388300


Now, in the world of Oracle, I would direct Oracle RDBMS to use direct I/O and avoid double buffering altogether. Is there something similar for PostgreSQL? In other words, may I somehow instruct PostgreSQL to use direct I/O and/or asynchronous I/O?

Mladen Gogala Sr. Oracle DBA 1500 Broadway New York, NY 10036 (212) 329-5251 www.vmsinfo.com The Leader in Integrated Media Intelligence Solutions